1. Field of the Invention
The present invention relates generally to time division multiple access transmission systems, and more specifically to a TDMA transmission system capable of high speed transmission of data and synchronization bursts higher than the operating speed of TDMA preambleless modems.
2. Description of the Related Art
In TDMA transmission systems a succession of short-duration bursts emanating from a number of different stations is presented to a demodulator. Each burst has its own independent carrier phase and consequently rapid-acquisition modems are mandatory. To permit rapid acquisition of a burst, a synchronization burst is first transmitted, followed by an information-carrying data burst. The synchronization burst is usually structured so that it starts with a preamble followed by a unique word and various control bits. The preamble contains a series of bits that accentuates carrier and clock line spectra to assist lock-up at the receive site during the training interval. However, the transmission of a preamble represents a loss of transmission efficiency, and hence proposals have hitherto been made to implement preambleless modems. At the transmit site of the prior art preambleless TDMA system, a preambleless TDMA sync burst is time-division multiplexed with a data burst of duration much longer than the duration of the sync burst, subjected to quadrature phase shift keying (QPSK) modulation upon an intermediate frequency carrier, and then to frequency translation to a microwave region for transmission. At the receive site, the transmitted bursts are frequency converted to baseband frequency, and stored into a buffer. A carrier and bit timing recovery circuit, connected to the buffer, performs a fast Fourier transform process on the stored data burst and detects the transmitted carrier and bit timing from the data burst. Using the recovered carrier and bit timing clock, the stored baseband signal is read out of the buffer into a QPSK demodulator where the whole contents of the original data and sync bursts are recovered collectively using what is known as an "en-block demodulation technique" and separated from each other by a time-division demultiplexer.
However, a dramatic transmission-efficiency improvement cannot be achieved by the prior art preambleless TDMA transmission since the preamble does not accounts for a substantial portion of the sync burst. Additionally, the amount of delays introduced by the read/write operations and the en-block demodulation process cannot be ignored for high speed TDMA transmission.